An inkjet type printer known in the art has a print head provided with a plurality of nozzle groups corresponding to inks of a plurality of colors and prints a color image by ejecting ink droplets of a plurality of colors onto a printing medium from the nozzle groups while scanning in a main scanning direction. This type of printer can print an image corresponding to a band-like unit printing region (band region) having the same width as a nozzle width (length of a nozzle group) in one scan of the print head. In order to print an image onto a printing region having a larger size than one band region, print of the image of the unit band region is repeated while shifting a position of the printing medium in a sub-scanning direction. Further, printing speed can be improved by executing bidirectional printing in which printing is performed by means of both a main scanning operation of the print head in a first direction and main scanning operation thereof in a second direction as compared to unidirectional printing in which printing is performed by means of a main scanning operation in only one direction. However, in the bidirectional printing, the same color in one image may be different between the main scanning operation in the first direction and main scanning operation in the second direction due to a difference in an ink overlapping order. As a result, an observed color may differ among observers. To suppress such a phenomenon, there is proposed a technique that calculates an index value concerning an estimated ink amount for each of a plurality of blocks in the band region and determines a specific one direction as the scanning direction when the index value is more than a threshold value.